Various home appliances such as televisions or refrigerators, or electronic devices such as laptop computers, portable phone terminals or Personal Digital Assistants (PDAs) operate with power that is received from the outside, and a built-in battery thereof may be charged with power supplied from an external power source. Furthermore, a power supply that converts a commercial Alternating Current (AC) voltage into a Direct Current (DC) voltage is built in home appliances or electronic devices (hereinafter generically referred to as electronic devices), or the power supply is built in an external power adaptor (AC adaptor) for the electronic devices.
The power supply includes a rectifier circuit (diode bridge circuit) that rectifies an AC voltage, and an insulating DC/DC converter that drops the rectified voltage and supplies the dropped voltage to a load.
FIG. 1 is a diagram illustrating a configuration of a DC/DC converter that has been studied by the inventor. A detailed configuration of the DC/DC converter 100r should not be regarded as a general technology that is well known to those skilled in the art.
An input port P1 of the DC/DC converter 100r receives a DC input voltage VIN from a rectifier circuit (not shown) that is prepared in a stage previous thereto. The DC/DC converter 100r drops the input voltage VIN, and supplies the dropped voltage to a load that is connected to an output port P2 of the DC/DC converter 100r. 
The DC/DC converter 100r mainly includes a switching transistor M1, a transformer T1, a first diode D1, a first output capacitor Co1, a control circuit 10r, and a feedback circuit 20r. In the DC/DC converter 100r, a primary side region and a secondary side region of the transformer T1 are necessarily required to be electrically insulated from each other. The feedback circuit 20r includes resistors R1 and R2 that divide the output voltage VOUT, a shunt regulator 22, and a photo coupler 24.
The shunt regulator 22 is an error amplifier that amplifies an error between the divided output voltage VOUT′ and a reference voltage VREF based on a target value of the output voltage VOUT. The photo coupler 24 feeds back a feedback signal, based on an error between the output voltage VOUT and the target voltage, to the control circuit 10r. The control circuit 10r controls an on/off duty ratio of the switching transistor M1 by using pulse modulation such that the output voltage VOUT matches with the target voltage.
The control circuit 10r may operate with a supply voltage VCC of about 10 V, and thus, when the control circuit 10r is driven with the input voltage VIN (about 140 V), efficiency is degraded. The voltage VOUT dropped by the DC/DC converter 100r is generated in the secondary side of the transformer T1, and thus cannot be supplied to the control circuit 10r that is prepared in the primary side.
Therefore, an auxiliary coil L3 is prepared in the primary side of the transformer T1. The auxiliary coil L3, a second diode D2, and a second output capacitor Co2 act as an auxiliary DC/DC converter for generating the source voltage VCC for the control circuit 10r. In the DC/DC converter 100r, the source voltage VCC is proportional to the output voltage VOUT, and a proportional coefficient is defined as a turn ratio of the secondary coil L2 of the transformer T1 and the auxiliary coil L3:VCC=VOUT×ND/NS where NS is the number of turns of the secondary coil L2, and ND is the number of turns of the auxiliary coil L3.